The present invention relates to a gas purification unit. The invention has particular application to the purification of air upstream of a cryogenic air separation system. Typically, the invention is used in conjunction with an adsorption process such as a temperature swing adsorption (“TSA”) process or pressure swing adsorption (“PSA”) process.
Where a feed gas is to be subjected to downstream processing, it may often be desirable or necessary to remove certain components from the feed gas prior to such processing. As an example, high boiling materials, e.g. water and carbon dioxide, which may be present in a feed gas, e.g. air, must be removed where the mixture is to be treated in a low temperature, e.g. cryogenic, process. If relatively high boiling materials are not removed, they may liquefy or solidify in subsequent processing and lead to pressure drops, flow difficulties or other disadvantages in the downstream process. Hazardous, e.g. explosive, materials should be removed prior to further processing of the feed gas so as to reduce the risk of build-up in the subsequent process thereby presenting a hazard. Hydrocarbon gases, e.g. acetylene, may present such a hazard.
In an air separation process, air is typically compressed using a main air compressor (“MAC”) and the resultant compressed air is cooled and fed to a separator where condensed water is removed. The compressed air may be further cooled using, for example, refrigerated ethylene glycol. The bulk of the water is removed in this step by condensation and separation of the condensate. The resultant substantially water-free air is typically then fed to an adsorption process, where the components to be removed from the air are removed by adsorption, and then to an air separation unit. In treating air, water is conventionally removed first and then carbon dioxide by passing the air though a single adsorbent layer or separate layers of adsorbent for preferential adsorption of water and carbon dioxide prior to feeding the air to the downstream separation process.
Several processes are known for removing an undesired component from a feed gas by adsorption on a solid adsorbent including TSA and PSA processes. Conventionally in such processes, two (or more) adsorbent beds are employed in parallel arrangement with one bed being regenerated “off-line” while the or each other bed is operated for adsorption. The roles of the beds are then periodically changed in the operating cycle. An adsorption bed is said to be “on-line” during the adsorption step.
In a TSA process, the adsorption step generates heat of adsorption causing a heat pulse to progress downstream through the adsorbent bed. The heat pulse is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-line period. After adsorption, the flow of feed gas is shut off from the adsorbent bed which is then depressurised. The adsorbent is then exposed to a flow of hot regeneration gas, typically a waste stream or other gas from a downstream process, which strips the adsorbed materials from the adsorbent and so regenerates it for further use. Regeneration conventionally is carried out in a direction counter to that of the adsorption step. The bed is then re-pressurised in readiness to repeat the adsorption step.
A PSA system typically involves a cycle in which the bed is on-line, and then depressurised, regenerated and then re-pressurised before being taken back on-line. Depressurisation involves releasing pressurised gas and leads to waste, generally known as “switch loss”. In PSA systems, the pressure of the regeneration gas is lower than that of the feed gas. It is this change in pressure that is used to remove the adsorbed component from the adsorbent. However, cycle times are usually short, for example of the order of 15 to 30 minutes, as compared with those employed in a TSA system which may be for example of the order of 2 to 20 hours.
Gas to be purified is usually fed to a gas purification unit, such as a vessel containing at least one adsorption bed, via inlet conduit means comprising a pipe and a flow control valve. Similarly, purified gas is usually removed from the gas purification unit via outlet conduit means comprising a pipe and a flow control valve. If the flow control valve of either the inlet conduit means or the outlet conduit means fails, then the flow of gas through the gas purification unit will be restricted (if the valve fails in a partially open position) or prevented (if the valve fails in the closed position) thereby reducing or completely stopping gas throughput through the unit.
It is known to bypass a control valve in the event that the control valve fails. For example, DE-C-195 06 760 discloses a PSA system in which the outlet end of the adsorber unit is connected via a common line to at least tour pressure balancing or purging lines which are isolated from the adsorber unit using open/shut valves. Control of the pressure balancing or purging lines is carried out via a first control valve in the common line. There is a reserve control valve arranged in parallel with the first control valve in a bypass line so that, even in the event of failure of the first control valve, “continued operation” of the PSA process is possible.
The pipes to and from the gas purification unit have to be rated proportionally to the capacity of the unit to allow the appropriate gas flow through the unit. It necessarily follows, therefore, that larger gas purification units require pipes having larger diameters than pipes to and from smaller gas purification units.
The size of the flow control valve must be appropriate to the size of the pipe with which it is associated. Butterfly valves are often used to control the gas flow through pipes to and from gas purification units. A large butterfly valve, e.g. one having a metal disc diameter of 100 cm. requires a powerful actuator to open and close the valve. The actuator must not only be able to move the large metal disc between the open and closed positions, but it must also be able to move the disc quickly and frequently during the adsorption/de-adsorption cycle of an adsorbent bed gas purification unit. For example, in a PSA process, the valve must be able to move from the fully open position to the closed position in about 1 or 2 seconds. Such powerful actuators are prone to breakdown as a result of, for example, bearing, seat or disc failure. Consequently, the reliability of a valve decreases as the size of the valve increases. In addition, the cost of a valve increases disproportionately as the size of the valve increases above a certain size.